BRUSSELS, May 25 (Reuters) - U.S. President Donald Trump on Thursday intensified his accusations that NATO allies were not spending enough on defense and said more attacks such as this week's bombing in Manchester would take place unless the alliance did more to stop militants.

In unexpectedly abrupt remarks as NATO leaders stood alongside him, Trump also said certain member countries owed "massive amounts of money" to the United States and NATO.

His scripted comments contrasted with NATO's choreographed efforts to play up the West's unity by inviting Trump to unveil a memorial to the Sept. 11, 2001, attacks on the United States at the new NATO headquarters.

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U.S. President Donald Trump and first lady Melania Trump arrive at the Brussels Airport, in Brussels, Belgium, May 24, 2017. REUTERS/Hannah McKay

U.S. President Donald Trump and first lady Melania Trump arrive at the Brussels Airport, in Brussels, Belgium, May 24, 2017. REUTERS/Jonathan Ernst

(L-R) U.S. Secretary of State Rex Tillerson, President Donald Trump and National security adviser H.R. McMaster eat Belgian chocolate during their meeting with Belgian Prime Minister Charles Michel in Brussels, Belgium, May 24, 2017. REUTERS/Jonathan Ernst TPX IMAGES OF THE DAY

European Council President Donald Tusk (L) speaks to US President Donald Trump (R) after welcoming him at EU headquarters, as part of the NATO meeting, in Brussels, on May 25, 2017. / AFP PHOTO / Emmanuel DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)

U.S. President Donald Trump (R) walks with the President of the European Council Donald Tusk in Brussels, Belgium, May 25, 2017. REUTERS/Francois Lenoir

U.S. President Donald Trump (R) arrives for his meeting with President of the European Council Donald Tusk and European Commission President Jean-Claude Juncker (neither pictured) at the European Union headquarters in Brussels, Belgium May 25, 2017. REUTERS/Jonathan Ernst

U.S. President Donald Trump (R) shakes hands with European Union foreign policy chief Federica Mogherini before their meeting at the European Union headquarters in Brussels, Belgium, May 25, 2017. REUTERS/Jonathan Ernst

U.S. President Donald Trump (R) and the President of the European Council Donald Tusk take their seats before their meeting at the European Union headquarters in Brussels, Belgium, May 25, 2017. REUTERS/Jonathan Ernst

U.S. President Donald Trump and first lady Melania Trump wait the arrival of French President Emmanuel Macron (unseen) before a lunch ahead of a NATO Summit in Brussels, Belgium, May 25, 2017. REUTERS/REUTERS/Jonathan Ernst

U.S. first lady Melania Trump greets French President Emmanuel Macron as President Donald Trump (C) looks on before a lunch ahead of a NATO Summit in Brussels, Belgium, May 25, 2017. REUTERS/REUTERS/Jonathan Ernst

U.S. President Donald Trump (L) and French President Emmanuel Macron shake hands before a lunch ahead of a NATO Summit in Brussels, Belgium, May 25, 2017. REUTERS/REUTERS/Jonathan Ernst

From L-R, Belgium's King Philippe, U.S. President Donald Trump, NATO Secretary General Jens Stoltenberg, German Chancellor Angela Merkel and Belgian's Prime Minister Charles Michel gather with NATO member leaders to pose for a family picture before the start of their summit in Brussels, Belgium, May 25, 2017. REUTERS/Christian Hartmann

U.S. President Donald Trump speaks beside NATO Secretary General Jens Stoltenberg at the start of the NATO summit at their new headquarters in Brussels, Belgium, May 25, 2017.REUTERS/Jonathan Ernst

U.S. President Donald Trump (L) and Britain's Prime Minister Theresa May react during a ceremony at the new NATO headquarters before the start of a summit in Brussels, Belgium, May 25, 2017. REUTERS/Christian Hartmann TPX IMAGES OF THE DAY

NATO Secretary General Jens Stolenberg (L), U.S. President Donald Trump and Britain's Prime Minister Theresa May (R) attend a ceremony at the new NATO headquarters before the start of a summit in Brussels, Belgium, May 25, 2017. REUTERS/Christian Hartmann

NATO country leaders including U.S. President Donald Trump (L) Canada's Prime Minster Justin Trudeau (L Top) and Britain's Prime Minister Theresa May (R) react during an aerial fly-pass at the new NATO headquarters in Brussels, Belgium, May 25, 2017. REUTERS/Christian Hartmann

U.S President Donald Trump (C) follows Britain's Prime Minister Theresa May as NATO member leaders gather before the start of their summit in Brussels, Belgium, May 25, 2017. REUTERS/Hannibal Hanschke

U.S President Donald Trump (C) takes his place as NATO member leaders gather before the start of their summit in Brussels, Belgium, May 25, 2017. REUTERS/Hannibal Hanschke

U.S. President Donald Trump (C) is flanked by British Prime Minister Theresa May (L) and NATO Secretary General Jens Stoltenberg during in a working dinner meeting at the NATO headquarters during a NATO summit of heads of state and government in Brussels, Belgium, May 25, 2017. REUTERS/Matt Dunham/Pool

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"We will never waiver in our determination to defeat terrorism and achieve lasting security, prosperity and peace," Trump said in the speech before a dinner with leaders.

"Terrorism must be stopped or...the horror you saw in Manchester and so many other places will continue forever," Trump said, referring to Monday's suicide bombing in the northern English city that killed 22 people, including children.

Trump called on NATO, an organization founded on collective defense against the Soviet threat, to include limiting immigration in its tasks as well as fighting terrorism and deterring Russia.

NATO leaders wanted Trump on Thursday to publicly support the military alliance that he had called "obsolete" during his campaign. But he instead returned to an old grievance about Europe's drop in defense spending since the end of the Cold War.

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"Twenty-three of the 28 member nations are still not paying what they should be paying for their defense," Trump said, standing by a piece of the wreckage of the Twin Towers.

"This is not fair to the people and taxpayers of the United States, and many of these nations owe massive amounts of money from past years," Trump said as the other leaders watched.

Praise was always going to be in shorter supply at the North Atlantic Treaty Organisation after Trump's sharp election campaign criticism of the alliance, which he blamed for not doing more to combat terrorism.

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US First Lady Melania Trump visits the Queen Fabiola children's hospital, on the sidelines of the NATO (North Atlantic Treaty Organization) summit, on May 25, 2017, in Brussels.
US President Donald Trump meets NATO and EU leaders for the first time with the US president set to press nervous allies to do more on terrorism after the Manchester bombing. / AFP PHOTO / Aurore Belot (Photo credit should read AURORE BELOT/AFP/Getty Images)

U.S. first lady Melania Trump visits the Queen Fabiola Children's University hospital in Brussels, Belgium, May 25, 2017. REUTERS/Hannah Mckay

US First Lady Melania Trump (C) visits the Queen Fabiola children's hospital, on the sidelines of the NATO (North Atlantic Treaty Organization) summit, on May 25, 2017 in Brussels.
US President Donald Trump meets NATO and EU leaders for the first time with the US president set to press nervous allies to do more on terrorism after the Manchester bombing. / AFP PHOTO / BELGA / LAURIE DIEFFEMBACQ / Belgium OUT (Photo credit should read LAURIE DIEFFEMBACQ/AFP/Getty Images)

U.S. first lady Melania Trump visits the Queen Fabiola Children's University hospital in Brussels, Belgium, May 25, 2017. REUTERS/Hannah Mckay

A girl takes a selfie with U.S. first lady Melania Trump at the Bambino Gesu hospital in Rome, Italy, May 24, 2017. REUTERS/Remo Casilli

U.S. first lady Melania Trump sits with Sara, (C) wife of Israeli Prime Minister Benjamin Netanyahu, talk to children during a visit to the Hadassah Ein Kerem hospital in Jerusalem May 22, 2017. REUTERS/Sebastian Scheiner/Pool

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Leaders had hoped for more, although a White House official insisted Trump, by being a member of the alliance, supported NATO's collective defense clause, which stipulates that an attack on one ally is an attack against all.

Before Trump spoke, Belgium's premier Charles Michel said it was time to "defend the values of the free world" while German Chancellor Angela Merkel said NATO was central to the West's security.

NATO still strived to impress Trump with military bands, allied jets flying overhead and a walk through the new glass-and-steel headquarters, which replaces a leaking, 1960s prefab structure.

Trump, a real estate magnate, called the building "beautiful" and joked that he did not dare ask how much it cost.

But it was left to NATO Secretary-General Jens Stoltenberg to carry through most of the planned pomp and to try to hammer home the message of unity.

"NATO is more than a club, more than an organization. NATO embodies the unique bond between Europe and North America," Stoltenberg said. "As we raise our flags today, our alliance stands strong united and resolute," he said.

In one nod to Trump, NATO leaders are due to agree later on Thursday for the Western military bloc to join the U.S.-led, 68-nation coalition against Islamic State in Syria and Iraq. (Additional reporting by Jeff Mason, Steve Holland, Sabine Siebold and Robert-Jan Bartunek)